In-plane Anisotropy of the Magnetic Fluctuations in NaxCoO2-yH2O

We report the $^{59}$Co NMR studies of the in-plane anisotropy of bilayer hydrated Na$_{x}\mathrm{CoO_{2}} \cdot$$y\mathrm{H_{2}O}$ using a oriented powder sample by a magnetic field in Fluorinert FC70. We found for the first time the $ab$-plane anisotropy of the $^{59}$Co NMR Knight shift $K$, the nuclear spin-lattice relaxation rate 1/$T_{1}$ and the nuclear spin-spin relaxation rate 1/$T_{2}$ at a magnetic field $H \sim$ 7.5 T up to 200K. Below 75 K, the anisotropy of $K$ is large compared with that at high temperatures. The hyperfine coupling constants seem to change around the temperature 150 K, in which the bulk susceptibility $\chi$ shows broad minimum, suggesting a change of the electronic state of CoO$_{2}$ plane. 1/$T_{1}$ also shows a significant anisotropy, which cannot be explained only by the anisotropy of the hyperfine coupling constants nor the anisotropic uniform spin susceptibility. The difference in the in-plane anisotropy of $T_{1}$ from that of $K$ indicates that the magnetic fluctuation at a finite wave vector $\vec{q} \neq 0$ is also anisotropic and the anisotropy is different from that at $\vec{q} = 0$.Comment: 4 pages, 5 figure

Large Magnetoresistance Effects in LnLnCoAsO with a Ferromagnetic-Antiferromagnetic Transition

A large magnetoresistance (MR) effect was observed in the layered compounds NdCoAsO and SmCoAsO, in which ferromagnetically ordered itinerant-electrons of Co are sandwiched by localized 4$f$-electrons of $Ln^{3+}$, below ferromagnetic-antiferromagnetic transition (FAFT) temperature $T_\mathrm{N}$ as observed in other FAFT compounds. In SmCoAsO, the large MR effect is also observed up to the Curie temperature $T_\mathrm{C}$, and it is found to be originating in the presence of another antiferromagnetic phase in the low-magnetic field region of the ferromagnetic phase.Comment: 5 pages, 5 figure

Weak Magnetic Order in the Bilayered-hydrate Nax_{x}CoO2⋅y_{2}\cdot yH2_{2}O Structure Probed by Co Nuclear Quadrupole Resonance - Proposed Phase Diagram in Superconducting Nax_xCoO2⋅_{2} \cdot yyH2_2O

A weak magnetic order was found in a non-superconducting bilayered-hydrate Na$_{x}$CoO$_{2}\cdot y$H$_{2}$O sample by a Co Nuclear Quadrupole Resonance (NQR) measurement. The nuclear spin-lattice relaxation rate divided by temperature $1/T_1T$ shows a prominent peak at 5.5 K, below which a Co-NQR peak splits due to an internal field at the Co site. From analyses of the Co NQR spectrum at 1.5 K, the internal field is evaluated to be $\sim$ 300 Oe and is in the $ab$-plane. The magnitude of the internal field suggests that the ordered moment is as small as $\sim 0.015$ $\mu_B$ using the hyperfine coupling constant reported previously. It is shown that the NQR frequency $\nu_Q$ correlates with magnetic fluctuations from measurements of NQR spectra and $1/T_1T$ in various samples. The higher-$\nu_Q$ sample has the stronger magnetic fluctuations. A possible phase diagram in Na$_{x}$CoO$_{2}\cdot y$H$_{2}$O is depicted using $T_c$ and $\nu_Q$, in which the crystal distortion along the c-axis of the tilted CoO$_2$ octahedron is considered to be a physical parameter. Superconductivity with the highest $T_c$ is seemingly observed in the vicinity of the magnetic phase, suggesting strongly that the magnetic fluctuations play an important role for the occurrence of the superconductivity.Comment: 5 pages, 6 figures, submitted to J. Phys. Soc. Jp